tests: do not use memstore.test_temp_dir in two tests

tests: do not use memstore.test_temp_dir in two tests

unittest_memstore_clone and test_objectstore_memstore.sh both
use memstore.test_temp_dir and when running in parallel they
interfere with each other.

src/test/objectstore/store_test_fixture.h will create

   memstore.test_temp_dir

when running via src/test/test_objectstore_memstore.sh

src/test/objectstore/test_memstore_clone.cc will also create

   memstore.test_temp_dir

when running via unittest_memstore_clone.

Fixes: http://tracker.ceph.com/issues/17743
Signed-off-by: Loic Dachary <loic@dachary.org>